An organic EL light source device (which may be referred to as an organic EL device) is a device that has an organic luminescent layer between a plurality of electrodes to achieve electrical luminescence. Organic EL elements are contemplated to be used as display elements as an alternative to liquid crystal cells. In addition, the organic EL elements are also contemplated to be used as surface light sources such as flat lighting devices and the backlights for liquid crystal displays, taking advantage of the characteristics such as high luminescence efficiency, low driving voltage, light weight, and low cost.
When an organic EL device is used as a surface light source, it is necessary to extract light from the organic EL device in a useful form at high efficiency. Specifically, even though the luminescent layer of the organic EL element itself has high light-emitting efficiency, the amount of light can be disadvantageously reduced before the light emission as the light passes through the multilayer structure that constitutes the element, due to, for example, interference in the layers. Therefore, there is a demand to render such loss of light as little as possible.
As a strategy for improving the light extraction efficiency, for example, Patent Document 1 (Japanese Patent Application Laid-Open No. 2004-296423 (counterpart publication: U.S. Patent Application Publication No. 2004195962)) discloses reduction of the brightness in a front direction (0°) of the device for increasing the brightness at angles in the range of 50 to 70°, whereby the overall brightness of a device is increased.